In its latest Safety Digest, UK MAIB describes an incident during which the towline of a harbour dredger failed, with the barge drifting free as a result.
The incident
A harbour dredger was on a coastal passage, relocating to start its next contract. The passage was to take several days and the dredger was towing an unmanned barge laden with an excavator, in an alongside configuration. There were four crew onboard but, with only one bridge watchkeeper, the plan was to stop each night for crew rest.
During the passage, the dredger encountered a long swell that made the alongside towing configuration untenable and so the crew switched to an astern tow; however, the towline soon failed and the barge drifted free. After the towline failed a second time, the crew decided to head for shelter.
In the approaches to the refuge port, the towline failed again and fouled both propeller shafts, disabling the dredger as the crew were attempting to recover the situation. Without propulsion, the dredger was anchored; however, the crew were unable to prevent a series of heavy collisions with the barge, which was then out of control. The collisions caused significant damage and flooding to the dredger.
The situation was eventually brought under control after the intervention of a lifeboat, an emergency towing vessel, and a powerful workboat that towed the dredger to safety for repairs.
Lessons learned
- Plan: Seagoing towing is a hazardous task that requires detailed planning and execution by capable crew. Although the dredger’s crew had experience of in-harbour towing and working with barges, they had little open sea towing experience. Post-accident analysis established that the crew’s tow plan had not identifed all the potential hazards with the passage and that the astern towing arrangements were vulnerable to failure. The key issues were that the towline length was too short and, without an elastic pennant or ‘stretcher’, it would be subject to potential overload due to ‘snatching’. There was also no chafe protection. The International Maritime Organization or trade association guidance should be reviewed when preparing towing plans. Additionally, schemes such as the Maritime and Coastguard Agency (MCA) recognised voluntary towage endorsement scheme can help to ensure suffcient crew competence when towing.
- Teamwork: The absence of a second watchkeeper was a signifcant limitation on a lengthy coastal passage and meant that a suitable port or anchorage had to be found every night; this was diffcult to arrange with uncertainties such as weather and the slow overall speed because of the tow. Historically, the dredger’s owner had employed a second watchkeeper for relocation passages or where there was a long sea passage to the spoil ground during dredging operations, but not on this occasion.
